FILTER. The smaller kinds of filters are strainers used in chemical operations for ren dering fluids transparent by separating the suspended impurities which make them turbid ; or for separating and washing the precipitates resulting from chemical analysis. They are usually made of unsized or blotting paper ; and they are used either spread out upon cloth stretched on a wooden frame, or folded and placed in funnels, and having conse quently the form of an inverted cone. They are either single or double, according to the purposes to which they are to be applied.
Various forms of filter are employed for the purpose of filtering water, either for drinking or culinary purposes. These filters generally depend upon passing water through sand or small pebbles and charcoal. It is well known that the Thames water, though it contains but little saline matter in solution, is frequently turbid, owing to mechanical admixture of earthy matter, which the filters in question are well calculated to remove, so as to render the water, though not so agreeable as spring water for drinking on account of its flatness, yet well adapted for other purposes.
A considerable portion of the river-water of Paris is filtered in large establishments where it is employed. The filters employed are small boxes, many in number, lined with lead, open at top, and having at the bottom a layer of charcoal between two layers of sand. If the water is foul, the upper layer of sand requires to be renewed daily. At the Hotel Dieu the boxes are hermetically sealed, and the water is forced through the filtering layers by arti ficial pressure.
A very simple water-filter may be made of a common garden-pot, or similar vessel, with a bottom pierced with holes. Fill the lower part with round pebbles, then place a layer of smaller pebbles, then coarse sand, and lastly a layer three or four inches in depth of well made pounded charcoal. The water, in per
colating through these various strata, loses nearly all its mechanical impurities.
Murray's Self-cleansing Domestic Tubular Filter,' registered in 1850, is to be soldered to the end of the service-pipe. The enlarged part of the pipe contains a perforated tube with several folds of flannel and linen wrapped round it. The smaller tap communicates only with the outer casing, so that no water can reach it that has not passed through the filtering tube. The larger tap communicates with the interior of the tube ; and by allowing it to run, the filter will clean itself.
In 'Bird's Hydrostatic Syphon Water Puri fier,' also registered in 1850, the filtration is performed in two inverted cones containing filtering media, situated in the cylinder. When used, the instrument is immersed in the water to be filtered, and the pipe uncoiled so as to hang with its stop-cock below the bottom of the instrument. On drawing out the air from the pipe, it acts as a syphon and a stream of pure water flows.
Foster's Pressure Filter, recently patented at Liverpool, and recommended for use by the Sanitary Board of that town, consists of a porous stone, hollow in the inside, and con tained in a metal jacket. This apparatus, when screwed on to the service-pipe, causes the water, forced through the stone bk the pressure of the main, to lose all its pollutions, and come out pure and clear in the extreme. There are two taps, one of which draws the filtered water from the interior of the stone globe ; the other the unfiltered from the ex terior; and the apparatus is so arranged that the drawing of the unfiltered water cleanses the stone and increases its powers of filtration.